Texture Paint Diffuse and Normal Map Over Existing Diffuse and Normal Map
I used two shaders input to a mix shader (cycles) and used two UV mappings - one is set to the default all faces overlapping for the tiled texture with normal map, and the other is for a smart UV unwrap for painting the stencil.

A normal Map node is for getting the bumps from the normal, and I chose blue for the original shader so it would contrast to see the effect.

Most people imagine that software like Substance Painter is projecting both the diffuse/albedo and the normal at the same time through the brush when it is actually similar to this set up using a stencil to reveal the automatic mapping.